1. Field of Invention
The present invention relates to a pump apparatus such as a fuel pump for use in an automotive vehicle to pump up fuel from a fuel tank and to supply the fuel into an internal combustion engine mounted on the vehicle.
2. Brief Description of Prior Arts
In the field of fuel pumps of the above kind, various types of fuel pumps have been known and practically used, such as displacement type pump (for example, roller pump), centrifugal pump, axial flow type pump and regenerative type pump. The regenerative type pump having a closed vane impeller is recently more often used since it can produce a high discharge pressure (2 to 3 kg/cm.sup.2) with less noise and pulsated discharge pressure, when compared with the roller pump.
However, since the fuel in a fluid passage of the regenerative type pump is agitated by a plurality of vanes of the impeller, vapor or cavitation is easily produced. Particularly when the regenerative type pump is used in a severe condition, such as at a high temperature and/or under a low ambient pressure, the vapor is produced and retained in a fuel pump chamber (in the fluid passage), causing a so-called vapor-lock with the result that the pump can not pump up the fuel even though the pump is operated.
To avoid such a vapor lock, it is known in the art, for example U.S. Pat. No. 3,418,991, a small vapor discharge port is formed in a pump housing for communicating a pump chamber with the outside, so that a certain amount of fuel always flows out of the pump chamber and when the vapor is produced it flows out of the pump chamber through the small discharge port.
According to the observations of the present inventors, a satisfactory result for avoiding the vapor lock can not be obtained when the small vapor discharge port is simply formed in the pump housing.